The present invention relates to a method for checking a value document having a polymer substrate and at least one see-through window as well as means for carrying out the method.
In this context, value documents are understood to mean sheet-shaped objects, which represent for example a monetary value or an authorization and thus shall not be manufacturable at will by unauthorized persons. They hence have features that are not simple to manufacture, in particular to copy, whose presence is an indication of authenticity, i.e. of manufacture by an authorized body. Important examples of such value documents are coupons, vouchers, checks and in particular bank notes.
The invention relates to checking a certain kind of value documents, namely those having a polymer substrate and at least one see-through window.
The value documents have a polymer substrate, which is understood to mean that they have at least one polymeric layer which serves at least partially as a carrier. Preferably, these are polymer bank notes.
Value documents having such polymer substrates mostly have on the surface of the polymer substrate one or preferably several areally applied cover layers opaque in the visible wavelength region; the cover layer or the cover layers are frequently printed on. With this or these there shall be achieved, inter alia, an appearance like that of a paper value document. Further, the uppermost cover layer serves, inter alia, as a carrier for printing inks with which the value document is printed. The value document needs to have in the visible wavelength region only an opacity that is usual with paper value documents, however. In particular, the value document having the cover layers may still be very weakly light-transmissive.
The value documents to be checked or checked within the framework of the invention further have a see-through window, which hereinafter is understood to mean a region of the value document which is transparent or translucent to optical radiation in a specified wavelength region, preferably in the visible spectrum, and has no opaque, areally applied cover layer. The see-through window, if transparency or translucency are given, may be a really printed by halftone printing with a transparent or translucent printing ink or also with any other printing inks or a material employed for forming the cover layer.
The see-through window may have any form and is limited by at least one areal region formed by the one or the several cover layers.
A problem of such value documents lies in the fact that these may lose printing ink or parts of the opaque, areally applied cover layer at individual places in the course of their use or circulation. For example, the printing ink or the opaque layer may chip off at these places. Such a deviation from a new value document caused by loss of the opaque layer and/or, where applicable, the printing ink located thereon will be referred to as a print removal within the framework of the present invention.
Such places are faults of the value document and possibly lower its usability or fitness for circulation. It is hence desirable, that value documents can be checked for such print removals. But the check is not simple, because there may be considerable variations between different print batches.